1. Field
Example embodiments generally relate to risk mitigation components used in nuclear power plants.
2. Description of Related Art
Nuclear reactors use a variety of damage prevention/mitigation devices and strategies to minimize the risk of, and damage during, unexpected plant events. An important aspect of risk mitigation is prevention of radioactive material escape into the environment. A containment building is conventionally constructed for this purpose to surround the reactor core, and several risk mitigation devices are used to ensure that the containment building is not breached during transient events.
A known damage and risk mitigation device is a Basemat-Internal Melt Arrest and Coolability device (BiMAC). A BiMAC is designed to prevent or reduce damage to the containment building in the case of a severe reactor accident involving reactor vessel breach and forceful relocation of molten core components to the containment building floor, or basemat. The ultimate purpose of the BiMAC, combined with several other layers of risk mitigation components and strategies, is to maintain containment integrity at least for 24 hours following the most probable severe nuclear plant accidents and, for known accident scenarios involving core-concrete interaction, reduce the likelihood of containment breach to 0.1% or less.
FIG. 1 is an illustration of a conventional containment building 10 cross section. Although containment 10 is shown in FIG. 1 having components and characteristics of an Economic Simplified Boiling Water Reactor (ESBWR), it is understood that components described therein are usable with other plant configurations. As shown in FIG. 1, containment 10 includes a reactor vessel 50 containing a core 55 filled with nuclear fuel bundles. A number of control blades 56 and control blade drives may be positioned below the core 55 and may be extended into core 55 to control the nuclear reaction therein. In an ESBWR, containment 10 may also include a gravity-driven coolant system 20, which may be a large, water-filled tank used to cool the core 55 in the event of a loss of primary coolant. Further, a suppression pool 25 may be within containment 10 and used to condense steam from vessel 50 and relieve pressure in the event of an accident.
A reinforced concrete containment vessel 15 surrounds the vessel 50 and several reactor components, so as to contain any radioactive materials that may escape from vessel 50 or other components during normal operation or during an accident. A lower drywell 60 is formed below the vessel 50 to house control blades 56, control blade drives, and core instrumentation and to provide space for core debris in the instance of vessel 50 breach or leak. For an ESBWR, the lower drywell 60 is largely circular with a diameter of approximately 11.2 meters. A liner 61 conventionally is placed over the concrete containment wall 15 in order to reduce corrosion and damage to wall 15 in the event of an hazardous material release in containment 10.
Basemat 62 is conventionally the lowest point of containment 10 and fabricated of similar materials as the reinforced concrete containment wall 15, directly above the ground. A BiMAC 100 may be placed above the basemat 62 to mitigate damage to basemat 62 in the event that molten or other core 55 debris is relocated to lower drywell 60, such as in the event of vessel 50 breach.
FIG. 2 is a detailed view of a conventional BiMAC 100 useable in ESBWRs. BiMAC 100 is described in Nuclear Regulatory Commission document NEDO-33201, Revision 2, “ESBWR DESIGN CERTIFICATION PROBABILISTIC RISK ASSESSMENT” (“NEDO-33201 Document”), which is incorporated herein by reference in its entirety. As shown in FIG. 2, BiMAC 100 may be placed immediately above basemat 62 and/or liner 61. Further, BiMAC 100 may line lower portions of the walls of the lower drywell 60.
A coolant supply line 65 may be connected to and may deliver coolant material to the BiMAC 100. Coolant may include a liquid having a high heat-absorption capacity, such as water. Coolant supply line 65 may be connected to a coolant source, and coolant may be delivered through coolant supply line 65 by a pump or other driving mechanism. Alternatively, coolant may be driven through coolant supply line 65 to BiMAC 100 by gravity alone, so as to be more fail-safe. For example, supply line 61 may be a lower drywell deluge line that connects to a pool of the gravity-driven coolant system 25 (FIG. 1) or other pool within containment 10. A fail-safe valve or other control mechanism, such as a squib valve, may initiate coolant flow through coolant supply line 65 to BiMAC 100 in the case of lower vessel 50 (FIG. 1) breach or other event.
BiMAC 100 includes a distributor line 120 that may connect to the coolant supply line 65 and/or other coolant source. Distributor line 120 may extend the entire length of the drywell 60 along basemat 62. Several parallel coolant channels 130 may extend, perpendicularly or otherwise, off of distributor line 120 at a 10-degree upward angle from the basemat. Coolant channels 130 may then extend up a portion of the lower drywell wall, where they terminate with an open end. In this way, coolant may flow into distributor line 120, feed into each coolant channel 130, and eventually flood into lower drywell 60. Distributor line 120 and coolant channels 130 may be fabricated of a material that substantially maintains its physical properties in an operating and transient nuclear reactor environment. For example, distributor line 120 and coolant channels 130 may be fabricated from a zirconium-based alloy, stainless steel, etc.
An ablation shield 110 may be placed over and/or may coat coolant channels 130 and distributor line 120. The ablation shield 110 may protect coolant channels 130 and distributor line 120 from thermal and chemical damage caused by molten core components forcefully relocating to lower drywell 60 in the event of vessel 50 breach. The ablation shield 110 may be fabricated from an inert, heat resistant, and conductive material, such as a ceramic or concrete. Additional shielding material 140 may be placed adjacent to coolant channels 130 to support the weight of core components relocated on top of BiMAC 100 during a vessel breach event. Additional shielding material 140 may be fabricated of a number of strong materials, such as concrete, ceramics, etc.
FIG. 3 is a detailed cross-sectional view of the coolant channels 130 of BiMAC 100. As shown in FIG. 3, channels 130 may be parallel and touch, so as to form a continuous wedge-shaped jacket of channels 130 capable of cooling the BiMAC and materials relocated thereon. Each channel 130 is 3.937 inches in inner diameter to provide sufficient coolant flow therethrough, as approved in the NEDO-33201 document. Ablation shield 110 may be formed directly atop and cover each channel 130, in order to provide heat conduction and cooling therethrough. Ablation shield 110 may be formed to different thicknesses than that shown in FIG. 3, depending on the material used to fabricate ablation shield 110 and the characteristics of the material to be cooled on top of ablation shield 110.
FIG. 4 is a top-down perspective view of BiMAC 100 illustrating operation of BiMAC 100 during an accident scenario. As shown in FIG. 4, during an initiating event, a valve is opened, permitting coolant flow down through a coolant supply line 65 to distribution line 120. Coolant flows into either end of distribution line 120 and then into coolant channels 130 up at a 10-degree angle toward walls 15. As molten or other hot debris is relocated to the lower drywell on top of an ablation shield 110 covering coolant channels 130, forced coolant flow through coolant channels 130 removes heat from the relocated debris, preventing continued melt and/or damage to basemat 62 and containment walls 15. Coolant exits the coolant channels 130 at a higher open end point of each channel 130, eventually flooding the lower drywell 60 and further aiding in cooling debris therein. As such, open end points of each channel 130 are typically located such that relocated debris cannot clog the channels 130. Further, if coolant flow to BiMAC 100 is provided by gravity, coolant flow and cooling may continue even if other plant mechanical systems fail that would otherwise be required to pump coolant into BiMAC 100, resulting in continuous, natural-circulation cooling.
The structure and function of BiMAC 100 described in FIGS. 1-4 has been extensively tested and submitted for approval to the Nuclear Regulatory Commission with dimensions of 3.937 inch inner diameter for coolant channels 130 and a 10-degree incline for coolant channels 130 with respect to basemat 62 in an ESBWR having an 11.2 meter diameter lower drywell 60.